This invention relates to a receiving apparatus and method, and more particularly to a receiving apparatus and method which can be suitably used to receive and directly orthogonally detect a digital modulated wave.
A broadcasting signal of a digital satellite broadcast such as, for example, the SKY PerfecTV (trademark) is a signal digitally modulated using the QPSK (Quadrature Phase Shift Keying). Accordingly, a receiving apparatus for receiving a digital modulated wave of the broadcasting signal includes an orthogonal detector which detects an I-component signal and a Q-component signal of the digital modulated wave.
FIG. 1 shows an example of a construction of a conventional orthogonal detector. Referring to FIG. 1, in the orthogonal detector shown, a digital modulated wave received by an antenna (not shown) and frequency converted into a signal of 950 MHz to 2,150 MHz in the L band is inputted as an input signal to an input terminal 1 of a preamplifier (AMP) 2. The preamplifier 2 amplifies the input signal and outputs a resulting signal to a variably controlled band-pass filter (BPF) 3. The variably controlled band-pass filter 3 removes an image interfering frequency included in the input signal thereto from the preamplifier 2 in response to a channel selection voltage from a low-pass filter built in a PLL (Phase Lock Loop) circuit 23, and outputs a resulting signal to an attenuator (ATT) 4. The image interfering frequency signifies a signal generated by heterodyne detection of a mixer 6 and having a frequency equal to an (oscillation frequency F1 of a local oscillator 22)+(intermediate frequency (480 MHz) outputted from the mixer 6).
The attenuator 4 limits the level of the signal from the variably controlled band-pass filter 3 to a fixed level based on an AGC (Automatic Gain Control) signal inputted thereto from the outside of the orthogonal detector, and outputs a resulting signal to an amplifier 5. The amplifier 5 amplifies the signal from the attenuator 4 and outputs the amplified signal to the mixer 6.
The mixer 6 multiplies the signal inputted thereto from the amplifier 5 by the signal inputted thereto from the local oscillator 22 to obtain an intermediate frequency signal (480 MHz) and outputs the intermediate frequency signal to an IF amplifier 7. The frequency (oscillation frequency) LF of the signal oscillated by the local oscillator 22 is determined with a control signal for controlling the dividing ratio of the PLL circuit 23 in response to a channel selection operation of a user so that the following expression (1) may be satisfied:
oscillation frequency LF=reception frequency Fin+intermediate frequency IF (480 MHz)xe2x80x83xe2x80x83(1)
In particular, output pulses of the voltage controlled oscillator built in the PLL circuit 23 corresponding to the control signal are integrated by a low-pass filter built in the PLL circuit 23 so that they are converted into a dc voltage. The dc voltage is supplied as a channel selection voltage to a resonance circuit (TANK) 24 and varies the voltage controlled variable capacitance of the resonance circuit 24 thereby to control the frequency of the signal to be oscillated by the local oscillator 22. Further, the channel selection voltage from the low-pass filter of the PLL circuit 23 is supplied also to the band-pass filter 3.
The IF amplifier 7 amplifies the intermediate frequency inputted thereto from the mixer 6 and outputs the amplified intermediate frequency to a SAW filter 8. The SAW filter 8 limits the frequency band of the intermediate frequency inputted thereto from the IF amplifier 7 and outputs a resulting signal to an IF amplifier 9. The IF amplifier 9 corrects amplitude loss of the SAW filter 8 and outputs a resulting signal to an attenuator 10. The attenuator 10 limits the level of the signal from the IF amplifier 9 to a fixed level based on the AGC signal and outputs a resulting signal to a pair of mixers 11 and 12.
The mixer 11 multiplies the signal from the attenuator 10 by a signal outputted from an oscillator 18 which is controlled by a SAW oscillator 17, and outputs a resulting signal to a baseband amplifier 13. The baseband amplifier 13 amplifies the signal from the mixer 11 and outputs the amplified signal to a low-pass filter 15. The low-pass filter 15 attenuates, among signals inputted thereto from the base band amplifier 13, those signals of frequencies higher than the intermediate frequency band. The low-pass filter 15 outputs a resulting signal as an I-(In-Phase) component signal from an output terminal 20.
The mixer 12 multiplies the signal from the attenuator 10 by the signal from the oscillator 18 having a phase shifted by 90 degrees by a 90xc2x0 phase shifter 19 and outputs a resulting signal to a baseband amplifier 14. The baseband amplifier 14 amplifies the signal from the mixer 12 and outputs the amplified signal to a low-pass filter 16. The low-pass filter 16 attenuates, among signals inputted thereto from the baseband amplifier 14, those signals of frequencies higher than the intermediate frequency band. The low-pass filter 16 outputs a resulting signal as a Q-(Quadrature-Phase) component signal from another output terminal 21.
It is to be noted that, for the I-component and Q-component signals outputted from the orthogonal detector, Viterbi decoding, error correction processing, decoding processing and so forth are thereafter performed successively.
While the orthogonal detector shown in FIG. 1 produces an intermediate frequency from an input signal of the L band and detects I-component and Q-component signals from the intermediate frequency signal, an orthogonal detector of the direct detection type has been developed in recent years. The orthogonal detector of the direct detection type is simplified in circuit construction such that it detects I-component and Q-component signals directly from an input signal of the L band.
FIG. 2 shows an example of a construction of an orthogonal detector of the direct detection type. It is to be noted that the orthogonal detector of the direct detection type is hereinafter referred to simply as direct orthogonal detector.
Referring to FIG. 2, in the direct orthogonal detector shown, an input signal of the L band amplified by a preamplifier 2 is inputted to an attenuator 4. The attenuator 4 limits the level of the input signal of the L band from the preamplifier 2 to a fixed level based on an AGC signal inputted thereto from the outside of the direct orthogonal detector and outputs a resulting signal to an amplifier 5. The amplifier 5 amplifies the input signal of the L band from the attenuator 4 and outputs a resulting signal to a pair of mixers 31 and 32.
The mixer 31 multiplies the input signal of the L band from the amplifier 5 by a signal outputted from an oscillator 37 and outputs a resulting signal to a variable gain amplifier 33. The variable gain amplifier 33 limits the level of the signal from the mixer 31 in response to the AGC signal and outputs a resulting signal to a low-pass filter 35. The low-pass filter 35 attenuates, among signals inputted thereto from the variable gain amplifier 33, those signals of frequencies higher than an intermediate frequency. The low-pass filter 35 outputs a resulting signal as an I-component signal from an output terminal 20.
The mixer 32 multiplies the input signal of the L band from the amplifier 5 by the signal from the oscillator 37 having a phase shifted by 90 degrees by a 90xc2x0 phase shifter 40 and outputs a resulting signal to a variable gain amplifier 34. The variable gain amplifier 34 limits the level of the signal from the mixer 32 in response to the AGC signal and outputs a resulting signal to a low-pass filter 36. The low-pass filter 36 attenuates, among signals inputted thereto from the variable gain amplifier 34, those signals of frequencies higher than the intermediate frequency. The low-pass filter 36 outputs a resulting signal as a Q-component signal from another output terminal 21. It is to be noted that the variable gain amplifiers 33 and 34 are provided to make up for limitation to the signal level which is insufficient where it is obtained only by the processing of the attenuator 4 and have a control width similar to that of the attenuator 4.
The oscillator 37 generates a signal of a predetermined frequency in accordance with a control signal for controlling the dividing ratio of a PLL circuit 38 in response to a channel selection operation by a user. In particular, output pulses of a voltage controlled oscillator built in the PLL circuit 38 corresponding to the control signal are integrated by a low-pass filter also which is built in the PLL circuit 38 so that they are converted into a dc voltage. Then, the dc voltage is supplied as a channel selection voltage to a resonance circuit 39 and varies the voltage controlled variable capacitance of the resonance circuit 39 thereby to control the frequency of the signal to be oscillated by the oscillator 37.
In this manner, since the direct orthogonal detector of FIG. 2 does not execute heterodyne detection (which corresponds to the processing of the mixer 6 of FIG. 1), image interference does not occur. Accordingly, since an image interfering frequency need not be removed, the band-pass filter 3 of FIG. 1 can be omitted from the direct orthogonal detector. Further, since several elements from the IF amplifier 7 to the attenuator 10 and so forth of FIG. 1 can be omitted from the direct orthogonal detector, the direct orthogonal detector is simplified in circuit scale and reduced in cost when compared with the orthogonal detector shown in FIG. 1.
An input signal to the direct orthogonal detector shown in FIG. 2 has a broad frequency range (950 MHz to 2,150 MHz). This gives rise to the following problem.
In particular, if an input signal to the preamplifier 2 is a single sinusoidal signal as given by the following expression (1),
Vi=(V cos xcfx89t)xe2x80x83xe2x80x83(1)
since the input/output nonlinear characteristic of the preamplifier 2 is such as given by the following expression (2),
V0=a0+a1Vi+a2Vi2+a3Vi3xe2x80x83xe2x80x83(2)
a second order harmonic of the input signal interferes with the reception band as indicated by the following expression (3),
V0=a0+a1V cos xcfx89t+a2(V cos xcfx89t)2+a3(V cos xcfx89t)3 =a0+a1V cos xcfx89t+a2V2(1+cos 2xcfx89t)/2+xe2x80x83xe2x80x83(3)
For example, when a signal of 1,900 MHz from input signals is to be received, a signal Vi of 950 MHz included in the input signals is amplified by the preamplifier 2, whereupon a higher harmonic component (cos 2xcfx89t) of a frequency of 1,900 MHz equal to twice that of the signal Vi is generated and interferes with the reception band.
Further, in the mixers 31 and 32, beat interference of a frequency Fb equal to the difference between a value equal to twice the frequency Fd of the signal to be received and the oscillation frequency LF of the signal from the oscillator 37 as given by the following expression (4) occurs:
Fb=2Fdxe2x88x92LFxe2x80x83xe2x80x83(4)
Where the frequency band of the input signal ranges from 950 MHz to 2,150 MHz as described above, the condition that the second order harmonic makes interference with reception is satisfied when one half the reception frequency Fd is 950 MHz or more as given by the following expression (5),
950 MHzxe2x89xa6Fd/2xe2x80x83xe2x80x83(5)
and the condition that the beat interference occurs is that twice the reception frequency Fd is 2,150 MHz or less as given by the following expression (6),
2Fdxe2x89xa62,150 MHzxe2x80x83xe2x80x83(6)
Accordingly, when the reception frequency Fd satisfies the condition of the expression (5), that is, when the reception frequency Fd is 1,900 MHz to 2,150 MHz, it is necessary to attenuate a signal of a frequency (950 MHz to 1,075 MHz) equal to one half the reception frequency Fd. Meanwhile, when the reception frequency Fd satisfies the condition of the expression (6), that is, when the reception frequency Fd is 950 MHz to 1,075 MHz, it is necessary to attenuate a signal of a frequency (1,900 MHz to 2,150 MHz) equal to twice the reception frequency Fd.
However, it is difficult to design a circuit by which the frequency band to be attenuated can be varied in response to a variation of a reception frequency as described above because the frequency range (950 MHz to 2,150 MHz) of the inputted signal is a broad band. Accordingly, there is a problem that it is difficult to prevent second order harmonic interference and beat interference in a direct orthogonal detector.
It is an object of the present invention to provide a receiving apparatus and method which can prevent second order harmonic interference and beat interference in a direct orthogonal detector.
In order to attain the object described above, according to an aspect of the present invention, there is provided a receiving apparatus for receiving and directly orthogonally detecting a digital modulated wave, including amplifying means for amplifying the digital modulated wave inputted thereto, attenuating means for attenuating, of the digital modulated wave amplified by the amplifying means, a signal of a frequency equal to twice a designated reception frequency or another signal of another frequency equal to one half the reception frequency, and detecting means for detecting an I-component signal and a Q-component signal from the digital modulated wave attenuated by the attenuating means.
According to another aspect of the present invention, there is provided a receiving method for a receiving apparatus for receiving and directly orthogonally detecting a digital modulated wave, including an amplifying step of amplifying the digital modulated wave inputted thereto, an attenuating step of attenuating, of the digital modulated wave amplified by the amplifying step, a signal of a frequency equal to twice a designated reception frequency or another signal of another frequency equal to one half the reception frequency, and a detecting step of detecting an I-component signal and a Q-component signal from the digital modulated wave attenuated by the attenuating step.
In the receiving apparatus and the receiving method, a digital modulated wave inputted is amplified, and from within the amplified digital modulated wave, a signal of a frequency equal to twice a designated reception frequency or a signal of another frequency equal to one half the reception frequency is attenuated. Then, an I-component signal and a Q-component signal are detected from the attenuated digital modulated wave.
Consequently, with the receiving apparatus and the receiving method, since a signal of a frequency equal to twice a designated reception frequency or a signal of another frequency equal to one half the reception frequency is attenuated, second order harmonic interference and beat interference can be prevented in a direct orthogonal detector.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.